The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on a GSM core network (CN) and Wideband Code Division Multiple Access (WCDMA) access technology. FIG. 1 illustrates a UMTS terrestrial radio access network (UTRAN) defined in the third generation mobile communications standard 3GPP.
As shown in FIG. 1, the UTRAN 110 includes one or more radio network sub-systems (RNSS) 120 and 130. Each RNS 120,130 includes Radio Network Controller RNC 121, 131 and one or more Node Bs 122, 123, 132, 133 (the Node B is similar to a radio base station). For example, Node B 122 is managed by RNC 121, and receives information transmitted from the physical layer of the user equipment (UE) 150 (sometimes called a mobile terminal) through an uplink channel and transmits a data to the UE 150 through a downlink channel. The Node B acts as an access point of the UTRAN from the UE's point of view. The RNCs 121 and 131 allocate and manage radio resources of the UMTS and are connected to a suitable the core network depending on types of services provided to users. For example, the RNCs 121 and 131 are connected to a mobile switching center (MSC) 141 for a circuit-switched communication, such as a voice call service, and are connected to a Serving GPRS Support Node (SGSN) 142 for packet switched communication such as a wireless Internet service. The RNC in charge of a direct management of the Node B is called a Control RNC (CRNC). The CRNC manages common radio resources. On the other hand, the RNC that manages dedicated radio resources for a specific UE is called a Serving RNC (SRNC). The CRNC and the SRNC can be co-located in the same physical node. However, if the UE has been moved to an area of a new RNC that is different from SRNC, the CRNC and the SRNC may be located at physically different places.
The UMTS includes interfaces that operate as a communication path between various network elements. For example, the interface between a Node B and a RNC is called an Iub interface, and the interface between RNCs is called an Iur interface. The interface between the RNC and the core network is called an Iu interface.
As wireless Internet services have become popular, various services require higher data rates and higher capacity. Although UMTS has been designed to support multi-media wireless services, the maximum data rate is not enough to satisfy the required quality of services. Therefore, the 3GPP is conducting research directed to providing an enhanced data rate and radio capacity. One result of the research is the High Speed Downlink Packet Access (HSDPA). The purpose of the HSDPA system is to provide a maximum data rate of 10 Mbps and to improve the radio capacity in the downlink.
Various techniques in the HSDPA system include Link Adaptation (LA) and Hybrid Automatic Repeat reQuest (HARQ). In the LA method, the UTRAN can choose the appropriate modulation and coding scheme (MCS) according to the channel condition. For example, if the channel condition is good, LA uses 16 Quadrature Amplitude Modulation (QAM) to increase the throughput. If a channel condition is not as good, however, LA uses Quadrature Phase Shift Keying (QPSK) to increase the probability of success.
The HARQ method retransmits lost packets, but the exact operation is different than the retransmission method in the RLC layer. If one packet is corrupted during transmission, HARQ transmits another packet that contains the additional information for recovery. The retransmitted packet and the original packet are combined in the receiver. The retransmitted packet may contain the same information as that of the previously transmitted data, or may contain any additional supplementary information for data recovery.
Since the HSDPA system is an evolutional form of the UMTS system, the UMTS network needs to be maintained as much as possible to support backward compatibility and to reduce the cost of network deployment. To reduce the impact of the changes, most of the HSDPA features are supported in Node B so that other parts of the UMTS network will not be affected. Accordingly, some functions in Node B need to be changed and some MAC functions are transferred from the RNC. The MAC functionalities constitute a new MAC sublayer in Node B and it is called “MAC-hs” sublayer. The MAC-hs sublayer is placed above the physical layer, and performs packet scheduling and various other functions (including HARQ and LA). In addition, the MAC-hs sublayer manages a transport channel called an HSDPA-Downlink Shared Channel (HS-DSCH), which is used to deliver data from the MAC-hs sublayer to the physical layer.
FIG. 2 illustrates a protocol structure of a radio interface protocol defined in the 3GPP. The radio interface protocol horizontally includes a physical layer, a data link layer, and a network layer, and is vertically divided into a control plane for transmission of control information (signaling) and a user plane for transmission of data information. The user plane is a region through which user traffic such as voice information or IP (Internet Protocol) packets is transmitted, and the control plane is a region through which control information required for network maintenance and management is transmitted.
The protocol layer structure 200 includes a Radio Link Control (RLC) layer 210, a Medium Access Control (MAC) layer 220, and a physical layer 230. The physical layer (PHY) handles transmission of data using a wireless physical channel between the UE and the UTRAN. The typical functions of the physical layer include data multiplexing, channel coding, spreading, and modulation. The physical layer exchanges information with the Medium Access Control (MAC) layer through a transport channel. A transport channel is classified as a dedicated transport channel or a common transport channel depending on whether its use is dedicated to one UE or whether it is shared among several UEs.
The MAC layer is further partitioned onto a MAC-d sublayer 222 and a MAC-hs sublayer 224. The MAC-d sublayer performs a set of functions that includes (1) mapping logical channels to common and dedicated transport channels, (2) multiplexing one or more logical channels onto one transport channel (C/T MUX), (3) ciphering/deciphering, and so on. The MAC-d sublayer 222 provides data flows to a MAC-hs sublayer 224 described further below, with each data flow being associated with certain scheduling attributes. The MAC layer transmits data by using a suitable mapping between logical channels and transport channels. The MAC-d sub-layer manages the dedicated transport channel, and a MAC-c/sh sub-layer which manages the common transport channel. The MAC-d sub-layer is located in the SRNC, and the MAC-c/sh sub-layer is located in the CRNC.
The radio link control (RLC) layer is responsible for reliable transmission of RLC protocol data units (PDUs). The RLC may segment or concatenate RLC service data units (SDUs) delivered from the higher layer. If the RLC PDUs are ready, they are delivered to the MAC layer and transmitted sequentially to the other node (UE or UTRAN). Sometimes, the RLC PDU can be lost during the transmission. In this case, the lost PDU can be retransmitted. The retransmission function of the RLC layer is called an Automatic Repeat reQuest (ARQ).
The RLC layer may include several RLC entities. Each of them performs an independent radio link control function. The operation mode of each RLC entity is one of a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM) depending on the adopted functions. In the RLC layer, data is processed as belonging to logical channels. There are various kinds of logical channels according to what kind of information the channel carries. The logical channel can be divided into two channels. One logical channel is a control channel for transmission of the control plane information and the other is a traffic channel for transmission of the user plane information.
The Packet Data Convergence Protocol (PDCP) layer is positioned over the RLC layer and efficiently transmits data of network protocols such as IPv4 or IPv6. For example, a header compression method in which header information of a packet is reduced can be used. The PDCP layer may include several independent PDCP entities like the RLC layer. The Broadcast/Multicast Control (BMC) layer is responsible for transmitting broadcast messages from a Cell Broadcast Center (CBS) positioned at a core network. The primary function of BMC is to schedule and transmit cell broadcast messages destined for a UE. The BMC layer, in general, uses an RLC entity operated in the unacknowledged mode in order to transmit broadcast messages. The Radio Resource Control (RRC) layer is a layer defined in the control plane. The RRC performs functions of establishment, reestablishment, and release of radio resources. In addition, the RRC layer can exchange control information between UE and UTRAN using RRC messages.
FIG. 3 illustrates a radio interface protocol for the HSDPA system. The MAC-hs sublayer is placed over the physical layer (PHY) in the Node B. In both the UE and UTRAN, the MAC-hs sublayer transfers data to the upper layer through MAC-c/sh and MAC-d sublayers. The MAC-c/sh and the MAC-d sublayers are located in the CRNC and the SRNC. A HS-DSCH Frame Protocol (FP) delivers the HSDPA data on the Iub or the Iur interface.
Radio link control (RLC) is typically operated in an acknowledged mode (AM) when used with HS-DSCH so that retransmissions are performed (when needed) between the radio network controller (RNC) and the user equipment (UE). Retransmission on MAC-hs level is also performed between the Node B and the UE for downlink data traffic. Since the Node B serves a large number of users, data to a particular UE for a particular priority data flow may need to be buffered in the Node B until it can be transmitted to that UE. If that UE uses several radio data flows with different priorities, low priority data may be buffered a significant period of time if higher priority data is available for transmission.
Because scheduling decisions are made at the Node-B in HSDPA, the Node-B must buffer data before transmission. The amount of data buffered can be negotiated with the RNC using a credit scheme. Of course, there is a certain delay associated with this negotiation procedure. The Node-B must send a capacity allocation message, which has to be processed by the RNC, and the associated data to be transmitted. This negotiation delay is referred to as “credit round-trip-time” (cRTT).
At any one time, the Node-B needs to store for each UE data flow enough data to satisfy all its transmissions that can take place during such a cRTT. Although it is possible to buffer enough data across all data flows for all data flows for all the UEs in the cell, this may lead to sub-optimal scheduling decisions driven by data availability rather than by channel conditions. If there is sufficient memory, better over-the-air performance is achieved if the Node-B distributes enough credits for all data waiting at the RNC. Acknowledged Mode (AM) RLC relies on retransmissions to achieve a desired residual frame error rate. Re-transmissions are triggered by sending feedback information on the status of each packet. The amount of buffering required in order to avoid “stalling”, i.e. the transmit and re-transmit buffers are full and cannot accept more data, is proportional to the over-the-air throughput and to the re-transmission round-trip-time (rRTT). The rRTT is the time between the time when a “hole” in the packet sequence numbers is detected by the receiver and the time when the packet is re-transmitted. It is desirable to reduce the rRTT to reduce the RLC buffer size or to improve RLC performance at equal RLC buffer size.
Currently, the Frame Protocol (FP) used between the HS-DSCH in Node B and in the RNC does not identify the type of RLC packet being sent down, i.e., whether the packet is a first time transmitted packet or a re-transmitted packet. This means that in addition to the status report transmission delay, the rRTT will also include buffering delays at the Node-B. The larger the amount of data sitting in the Node-B buffer, the longer the rRTT.
The buffering delay in Node B affects the RLC retransmission round trip time (rRTT) and negatively impacts the RLC performance in terms of delay and throughput. The rRTT is reduced by introducing different priorities for different types of RLC packet data units (PDUs) in a single data flow. For example, “status” PDUs, i.e. ARQ feedback information transmitted in the downlink direction to the UE, have a high priority. For uplink traffic, performance is improved by transmitting the RLC status PDUs as quickly as practical in the Node B. As another example, “retransmitted” PDUs are given a higher priority than PDUs transmitted for the first time. For downlink traffic, prioritizing retransmitted PDUs over PDUs transmitted for the first time permits faster delivery of UE data to higher protocol layers. Because AM RLC uses in-order delivery, data is delivered in the same order it was transmitted from the RNC RLC entity. If an RLC PDU is missing, all PDUs with higher sequence numbers are buffered until the missing PDU is received. Thus, a missing PDU causes delay for all subsequent data. Performance is improved by assuring that the missing PDUs, i.e. retransmissions, are prioritized.